In recent years, with computers and information communication equipments developing toward high performance, high function and networking, to transmit and process large-capacity information at high speed, operation signals tend to be high-frequency. Therefore, the material of the circuit substrate material is required to be improved.
Among the conventional materials used for a printed circuit substrate, epoxy resins with excellent adhesion characteristics are widely used. However, an epoxy resin circuit substrate generally has relatively high dielectric constant and dielectric loss tangent (the dielectric constant is more than 4, and the dielectric loss tangent is around 0.02), and inadequate high-frequency characteristics, which result in failure in meeting the requirement of high-frequency signals. Therefore, it is required to develop resins with excellent dielectric properties, that is, resins having low dielectric constant and dielectric loss tangent. For a long time, technicians in the field have studied thermosetting polybutadiene or copolymer resin of polybutadiene and styrene, which have excellent dielectric properties. The results of these studies are cited as the following.
PCT patent application (WO97/38564) disclosed a circuit substrate made from nonpolar styrene-polybutadiene divinyl benzene terpolymer with magnesium aluminum silicate as a filler and fiberglass cloth as a reinforcing material being added in, which has excellent dielectric properties but poor heat resistance. The glass transition temperature of the circuit substrate is only about 100° C., and the coefficient of thermal expansion is very large, so it is hard to meet the high temperature (over 240° C.) requirement in the lead free process of manufacturing PCBs.
A US patent (U.S. Pat. No. 5,571,609) disclosed a circuit substrate made from 1,2-polybutadiene resin or polyisoprene with molecular weight being less than 5000, polybutadiene styrene copolymer with high molecular weight, lots of silica as a filler, and fiberglass cloth as a reinforcing material, which has excellent dielectric properties. However, in the patent, a component with high molecular weight is adopted to improve the sticky property of the prepregs, so the processing property of the process of manufacturing prepregs becomes bad. Moreover, the proportion of rigid structure benzene ring in the resin molecule of the whole resin system is very small, and the cross-linked segments are mostly composed of methylene with very low rigidity. So, the made sheet has poor rigidity and very low flexural strength.
A US patent (U.S. Pat. No. 6,569,943) disclosed a circuit substrate made from amine modifier polybutadiene liquid resin with vinyl at the molecule terminal, lots of monomers (styrene dibromide) of low molecular weight as a curing agent and diluent, and impregnate fiberglass cloth, which has excellent dielectric properties. However, for the resin system is liquid at normal temperature, it can not be made into non-sticky prepregs. Therefore, in the compression moulding of sheets, it is difficult to adopt common stacked technology of prepregs, which results in difficult process operation.
Chinese patent No. CN1280337C used polyphenylene ether resin with unsaturated double bonds at the molecule terminal, and adopted vinyl monomer (such as styrene dibromide) of low molecular weight as a curing agent. However, since these monomers of low molecular weight have a low boiling point, which will volatilize during the drying process of impregnating fiberglass cloth to make a prepreg, the usage amount of the curing agent can not ensure to be enough. Besides, although the patent disclosed that polybutadiene resins can be used to improve the viscosity of the system, it did not clearly disclose that polybutadiene resins with polarity groups were adopted and polybutadiene resins with polarity groups were adopted to improve the peel strength.
The above said patents or patent applications still have a conspicuous problem, that is, the peel strength of the made copper cladded sheet is low (less than 0.8 N/mm), which causes the made copper cladded sheet to have the risk of the circuit peeling off while processing PCB circuits or using the equipments.